Sandwich structures made of composite material present exceptional properties, especially mechanical. They combine high resistance to mechanical and/or thermal constraints and high rigidity for a minimum mass.
These structures are therefore utilized widely in the space industry (satellites, probes, launchers) and aeronautical industry (radome, hatches, leading edges, ailerons, etc).
However, it is known that these structures are vulnerable to lightning strikes if they are not properly grounded. The high-density electric currents that then cross these composite structures can seriously damage them and lead, for example, to delamination. The case of the radome is even more critical since, by definition, it cannot integrate the standard lightning current discharge devices, such as a metal grid inserted in the structure's surface that, because of the requirements for transparency with regard to radar waves, are not allowed.
Means are known allowing the lightning's energy to be dissipated so as to protect these structures in the event of a lightning strike.
FIG. 1 shows such a means of dissipating the lightning energy according to the prior state of the art. Lightning conductor strips 1, typically made of aluminum or copper, are installed on the external surface 2 of an airplane's radome 3. These strips 1 are fastened on the radome by screw-type fastener units 4. Each lightning conductor strip 1 is electrically linked to the external surface 2 of the radome 3 to allow the electrostatic charges that build up there to be discharged. The member 5 in which the screw 4 is fitted to fasten the strip 1 is made of insulating plastic.
Each strip 1 is also connected to the junction of the airplane's fuselage to ensure that it is grounded individually. Thus a lightning arc hitting this strip 1 has its energy evacuated towards the ground without affecting any other element of the radome.
Although this energy dissipation means gives good results, perturbations 6 in the flow of the air 7 incident on the radome 3 have been observed, which are caused by the protrusions formed by the lightning conductor strips 1 at the external surface 2 of the radome (FIG. 1b).
However, when it constitutes an airplane's “nose” the radome has a generally conical shape to ensure good penetration in the air, and as a result makes a significant contribution to the aerodynamics of this airplane.
These aerodynamic perturbations 6 generate a drag, increased in particular by the triggering of the transition from laminar to turbulent flow, and consequently a very noticeable rise in the airplane's consumption of fuel, which is incompatible with the economic requirements of the airlines.
Further, when there is a severe lightning strike on a lightning conductor strip, damage to it can be observed with, for instance, a superficial fusion of this strip's fastening screw. Maintenance of the radome thus requires the latter's removal so that the damaged strip can be removed and replaced.
It would therefore be beneficial to have a system protecting an airplane's radome against lightning allowing drag to be reduced so as to produce a significant saving in fuel weight while having the greatest transparency possible with regard to radar waves.